This invention relates to a method and apparatus for separation of the liquid and solid phases of a slurry, such as a calcium carbonate containing mud slurry.
The invention is an improvement over U.S. Pat. No. 5,143,555 issued to Brady and U.S. Pat. No. 3,521,751 issued to Holthuis and EPO Application 454,392 by A. Ahlstrom Corp., all incorporated by reference.
When a vacuum drum is employed to separate the liquid from the solid phase of a two phase mixture, such as a mud slurry, with the drum partially submerged and rotating on a horizontal axis in a slurry vat, a cake of the solid phase material builds up on the porous outer surface of the vacuum drum. A filter cloth is commonly used. One or more knives, termed doctor blades, typically extending the full width of the drum, continuously scrape off a portion of the solid phase or cake, with the scraped off portion falling into the slurry vat in which the drum rotates, for subsequent removal. However, the doctor blade does not continuously scrape all the caked material off of the vacuum drum surface because the blade does not contact the drum surface.
With continued build up of the cake, the separation process must be halted from time to time and the vacuum drum stopped and backwashed, from its inside to its outside, to dislodge the cake from the outer surface of the drum. This stoppage represents a loss of time and is hence more expensive than continuous operation.
The noted Brady patent describes an improvement on conventional vacuum drum and knife arrangements, the Brady method and apparatus employing a second and shorter doctor blade or knife, of substantially less width than that of the drum, and which periodically goes back and forth along the width of the drum, above the stationary doctor blade, to partially scrape off the mud cake. The Brady apparatus and method also requires backwashings but increases the time between them.
The noted Holthuis patent discloses a submerged cake dislodging jet of water periodically movable back and forth along the width of the vacuum drum, the nozzle for the jet located within the slurry vat. The Holthuis nozzle is located beneath the surface of the slurry which is to be separated into its components. However, if agitators are provided in the slurry vat, it would be difficult to incorporate such a submerged nozzle. Further, the location of the nozzle, in the slurry containing vat and below the slurry surface, is an impediment to retrofitting existing vacuum drum installations as well as for nozzle repair or replacement.
Above noted European Patent Application 454,392 shows an arrangement similar to Holthius, except that two reciprocating nozzles are above the slurry level, yet are also positioned within the vat.
In both Holthius and the EPA apparatus, the nozzle discharges a water jet orthogonal to the rotating drum surface. This has a tendency to push the cake back into the filter cloth.